Oil recovery from oil shales by transverse combustion



y 6, 1970 E. H. BRUIST 3,513,913

OIL RECOVERY FROM OIL SHALES BY TRANSVERSE COMBUSTION Filed April 19.1966 COMBUSTION COMBUSTION SUPPORTING PRODUCTS SUPPORTING FLUIDS 4 FIG.I

conausnon conausnon supromme PRODUCTS SUPPORTING 2 FLUIDS 3 1' FLUIDSOVERBURDEN FIG. 2

INVENTORZ EDMON D H. BRUIST HIS AGENT United States Patent 3,513,913 OILRECOVERY FROM OIL SHALES BY TRANSVERSE COMBUSTION Edmond H. Bruist,Bakersfield, Calif., assignor to Shell Oil Company, New York, N.Y., acorporation of Delaware Filed Apr. 19, 1966, Ser. No. 543,698 Int. Cl.EZlb 43/24 US. Cl. 166-245 7 Claims ABSTRACT OF THE DISCLOSURE A processof recovering hydrocarbon products from subterranean oil shaleformations by drilling at least two spaced boreholes into the formationin a manner such that the lower terminal ends of the boreholes approachconvergence adjacent to the bottom of the formation. Sufiicient oilshale is removed in the area of convergence to form a shale expansionchamber communicating at all times with the lower terminal ends of theboreholes and in situ combustion is established in the chamber. Acombustion-supporting fluid is supplied through at least one of theboreholes to maintain the combustion within the chamber and cause acombustion front generally parallel to the bedding planes of theformation to move vertically and upwardly through the formationgenerally perpendicular to the bedding planes of the formation andbetween the converging boreholes. Finally, vaporous efiluent produced inthe combustion is recovered from the formation through at least one ofthe boreholes.

This invention relates to the recovery of hydrocarbon products from oilshales and, more particularly, to a method of recovering hydrocarbonsfrom underground shale deposits by a novel in situ retorting technique.

Looking to the future, oil shales are essentially thought of as animportant energy source as the reserves of petroleum crudes dwindle.Even today in those countries which are deficient in conventionalcrudes, such as Great Britain and Sweden, hydrocarbons from oil shalesare being presently produced. Since it has proved feasible to extracthydrocarbons or hydrocarbon products from oil shales on an industrialscale, producers look critically at oil shales for replacement of thedwindling supply of crude petroleum. Further, the availability ofenormous deposits of oil shales in various parts of the world, which isestimated at 1.2 to 2 trillion barrels, warrants considerable expenseand development of better techniques to recover hydrocarbon productsfrom these vast oil shale deposits.

Organic matter in these vast oil shale deposits is often referred to askerogen and is unlike either petroleum or coal types of organic matter.Further, oil shales are an intermediate hydrocarbon-bearing substancehaving properties considerably different than either petroleum or coal,since the oil shales contain appreciable amounts of inorganic matter,generally well above 33%, as distinguished from coal which contains veryminute amounts and petroleum which is substantially free of inorganicmatter. Oil shales should not be classified with rocks which actuallycontain impregnated petroleum, such as the Athabasca sands (tar sands)of Alberta, Canada, and represent a separate, distinct source ofhydrocarbons having sharply contrasting physical characteristics.

In recovering the organic matter or hydrocarbons from oil shales, onemust remain cognizant of the physical characteristics of the shale. Oilshale, is usually a dense, tough, resistent and impermeable substance.In general, the oil shales are regularly bedded and, with fewexceptions, thinly laminated. Rhythmatic laminations are by far the mostfrequent type and are due to regular alternation of 3,513,913 PatentedMay 26, 1970 micro-granular layers of carbonate and clay with layers ofstructureless organic matter. In some high grade varieties of oilshales, the lamination is not apparent until after the rock has beenheated and the hydrocarbon products driven off. However, closeexamination will generally reveal the laminar characteristics of theshales and the bedding planes are generally fiat except wheredeformation has occurred caused by tectonic movement.

Many investigators have concluded that commercial production ofhydrocarbons from oil shales should be handled in a retorting typeprocess using mined shale which is crushed or broken up before theretorting is carried out. Since as much as of the material handled willbe waste mineral matter, the logistical problems in handling the minedshale become enormous. Further, waste disposal problems of the spentshale become diflicult and finely divided spent shale is difficult tohandle. In addition, the natural leaching of the spent shale in a largedumping area can cause serious ground water pollution problems whichrequires that the handling of spent shale be carefully controlled. Inview of these problems, in situ retorting of the oil shale hasconsiderable inherent value since the expense of materials handling isalleviated and the disposal problems of spent shale is avoided.Likewise, mechanical equipment used to dig, mine, haul and crush theshale are not required and these factors reduce the costs of in situcombustion processes provided such processes yield feasible recovery.

In situ retorting deserves special consideration in view ofmany'advantages mentioned above. To date, the retorting of oil shale insitu is generally divided into two classes. In one type, relativelylarge drift-size mine workings were dug into a flatlying oil shale afterwhich the walls of the drift were blasted to fill the drift with brokenshale. Then, by closing off the openings at one end with suction devicesand applying combustion-supporting fluids at the opposite ends, it ispossible to burn the broken shale filling the drifts and recoverhydrocarbons therefrom. In this technique, quantitative recovery is lowand performance was generally poor in many respects. Probably one of themain difficulties of such a process is the necessity to remove prior totreatment a quantity of shale equal to about a third of the amount ofshale treated to make room for the broken rock filling the drifts duringtreatment of the shale.

A second type in situ combustion is described in US. Pat. 3,149,670issued to Grant which is carried out by drilling two closely spacedboreholes, establishing communication therebetween, and carrying out anin situ combustion process where the heat front moves from one well tothe other. See also US. 2,780,449 issued to Fisher et al.

Processes such as those mentioned in the patents have been onlymoderately successful caused by the lack of primary permeability of theoil shales. They require combustion fronts moving through the formationfrom injection to production wells utilizing artificially inducedfractures and in this manner fail to utilize fully the secondarypermeability created in the burned region as a result of the laminarcharacter of oil shales. The present invention is a vast improvementover those known prior art processes in that it employs a generallyhorizontal combustion front at all times extending from injection toproduction well(s) which moves vertically from bottom to top of theformation through the oil shale. This type of a combustion front,extending generally parallel to the bedding planes and moving throughthe oil shale in an upward direction more or less perpendicular to thebedding planes, has substantial advantages since the combustion front ismore easily maintained and the flow of combustion-supporting fluids cantravel along and be- 3 tween the laminates of the oil shale aspermeability is created by splitting or spalling of the shale due to theburning and heating effect of the combustion reaction.

More specifically, the instant invention involves an in situ combustionprocess in underground oil shale formations which includes the steps of(1) drilling at least two spaced boreholes into the shale formation in amanner that their lower terminal ends will approach convergence at thebottom of the reservoir, (2) removing oil shale in the area ofconvergence to form a chamber having communication with the lowerterminal ends of the boreholes, (3) establishing in situ combustionwithin the chamber, (4) supplying combustion-supporting fluids via oneof the boreholes, and (5) recovering hot vaporous efiluents from theformation produced by the in situ combustion via one of the boreholes.Utilizing this method, a generally horizontal combustion front isestablished in the bottom of the shale formation which burns upwardlythrough the formation with increasing radius and at all times extendingfrom injection to production well(s).

By the use of this novel method of in situ combustion, several of themajor difiiculties experienced in the in situ combustion process forrecovery of hydrocarbons from oil shales can be avoided. Probably, themajor problem overcome is that of maintaining fluid communicationbetween the spaced boreholes used in such processes, which, inconventional processes, requires high gas pressures to force hot gasesthrough the artificially created fractures connecting the boreholes.Further, as the hot gases pass through such fractures, they tend toclose because of the expansion of the unspent shale heated by exhaustgases and the entry into the fracture of viscous petroleum products.Closer spacing of the boreholes helps these problems, but is notpractical from the economic point of view. It should be appreciated thatin the instant invention, there is no problem in the closing offractures or the necessity of high gas pressures to force gases throughthe shale since both. the injection boreholes supplying thecombustion-supporting fluids and the production boreholes for recoveringthe hydrocarbon prod ucts all communicate with a common cavern in thebase of the oil shale reservoir. Further, the curvature of the boreholesprovides satisfactory borehole spacing controlling the lateraldimensions of the front and making the process practical from aneconomic standpoint.

Since the combustion-supporting fluid is supplied to the combustionfront burning from the bottom of the reservoir toward the top from oneor more wells located generally around the periphery of the cavern, thecombustin-supporting fluid to the combustion front is suppliedtransverse to the vertical movement of the front. Thus, this process ofin situ combustion recovery of oil shales can be termed a transverse insitu combustion process, since the combustion-supporting fluid issupplied across the combustion front rather than perpendicular to it asin prior art processes.

The general description above will be more easily understood byreference to the specific description covering the drawings wherein:

FIG. 1 shows a cross-section of an earth formation having a subterraneanoil shale stratum penetrated by several boreholes, and

FIG. 2 is the same cross-sectional view of the same formation showingconditions in the reservoir subsequent to the initiation of the recoveryprocess according to this invention, and at an intermediate stage.

Referring to FIG. 1 and 2, which show the same reservoir wherein theprocess of this invention is being employed, FIG. 1 shows thepreparation of the reservoir for initially carrying out the process andFIG. 2 shows an intermediate stage of the process as it is being carriedout in the reservoir. While in actual practice, a plurality ofconverging boreholes would be employed to achieve better results, theprocess can be amply illustrated by the three borehole pattern shown inthe FIGS. 1 and 2. Also, it should be appreciated that the process couldbe car- 4 ried out by the use of only two converging boreholes. However,three or more are usually preferred.

As illustrated in FIG. 1, three boreholes are drilled through theoverburden 1 into an underground shale reservoir 2; the central borehole3 is drilled vertically down into the reservoir 2 and is cased throughthe overburden with casing 4 which is sealed in the overburden with asealant 5, such as cement. Borehole 3 extends vertically downward intothe shale reservoir 2 terminating near the bottom reservoir and isuncased below the overburden 1. The two outer boreholes 7 and 6,respectively, laterally spaced from the central borehole 3, are alsocased only through the overburden with casings 4 which like the casing 4of borehole 3 are secured in the overburden with a sealant 5.

The lower portions 8 and 9 of boreholes 6 and 7 are not cased and aredirectionally drilled so as to approach convergence or actually convergeat the base of the shale reservoir 2 and in the vicinity of borehole 3.Actually, the art of directional drilling is well known and competentdrillers can hit a target area about 10 feet square in the base ofreservoir 2 without undue difficulty. Tools for directionally drillingboreholes are covered by numerous U.S. patents, for example see U.S.2,669,430 issued to Zubin and U.S. 2,906,499 issued to Travis.Occasionally, the converging boreholes 8 and 9 may actually intersectwith one another or borehole 3 when drilled. However, this is notnecessary for the practice of this invention. Generally, it is onlynecessary to hit a target area in the base of shale reservoir 2 so thatthe respective ends of boreholes 3, 5 and 6 will be within such adistance of one another that explosives or fracturing fluids can beintroduced through the respective boreholes to effect a fluidcommunication between them. The distance will be preferably within 10 or20 feet of one another however larger distances may be satisfactorywhich depends on the physical characteristics of a particular oil shale.

Conventional fracturing techniques with fluids, explosive or nucleardevices can be used to establish fluid communication between the lowerterminal ends of the boreholes 3, 5 and 6 in the target area located atthe base of oil shale reservoir 2, and in most cases, such fracturingwill usually be necessary where the several boreholes do not actuallyintersect during the drilling operation because of the fluidimpermeability of oil shale formations.

After fracturing has been completed, a cavern 10 is formed within thetarget area to connect the various boreholes with one another and toprovide space for the initial expansion of the unspent oil shale as itis heated by the combustion reaction. Normally, chamber 10 can be formedby conventional techniques, such as the use of acids which will dissolvethe carbonates, drilling, bailing or other removal techniques to removea quantity of the shale within the target area at the base of reservoir2 or drilling devices for enlarging the terminal ends of the boreholesadequately to provide suflicient shale removal to practice theinvention.

A preferred sequence of operations may be to drill the central borehole3 first, set off a large explosion in the bottom of the hole and followthis by drilling the directional boreholes 6 and 7. When the latterholes reach the area where the rock has been shattered by the explosion,the fractured oil shales will be drilled up and circulated to thesurface and the required cavern 10 will be formed. If necessary,additional explosions or leaching with acids followed by cleaning outwith a drill bit can be employed. It will be desirable, however, thatthe cavern be at least partly filled with crushed oil-shale as this willfacilitate ignition. In addition to crushed shale, a propping agent suchas sand or gravel can be placed in the cavern to prevent closing of sameby the expanding shale when heated.

The maximum space initially required can be calculated from theexpansion coefiicient and the heat conductivity of the oil-shale and thetemperature and areal extent of the combustion front during ignition.

In some cases, the use of chamber may not be required where the severalboreholes are terminated within a very small target area so thatsubsequent expansion of the unspent oil shale will not close off thecommunication between their respective terminal ends. However, in mostcases, because of the excessive expansion of oil-shales when they areheated, it will be necessary to form chamber 10 at the base of formation2 in order to start and maintain the combustion front satisfactorily.

After cavern or chamber 10 has been formed at the base of the reservoir2 in the target area and provides substantial communications between theseveral boreholes penetrating the shale formation 2, the second phase ofthe invention may be carried out which can be better understood byreference to FIG. 2. Initially, it is necessary to establish combustionwithin chamber 10 at the base of the shale reservoir 2 which can beaccomplished in numerous ways. For example, a burner assembly may beintroduced into chamber 10 and supplied with fuel andcombustion-supporting fluids from any one of the several boreholes. If aheater is used, it would probably be lowered in borehole 3 since thiswould be the most convenient, but the combustion can be established inmany other ways. For example, pyrotechnical devices can be introducedinto the chamber 10 and both combustionsupporting fluids and combustiblefluids supplied to the chamber through the boreholes 3, 6 or 7. Once thecombustion reaction has been established in chamber 10, the naturaltendencies of heated materials to rise will cause the greatertemperature to occur along the top of chamber 10 and for the most part,a generally horizontal combustion front will be formed across the top ofchamber 10 though some burning will occur at other locations.

After the combustion front is established, the unspent oil shale aroundchamber 10 will begin to expand but since chamber 10 is void of or onlypartly filled with crushed and broken oil shale, this expansion will notbe sufiicient to close chamber 10 or impede fluid communication betweenthe terminal ends of all the boreholes communicating therewith. Thus,the expansion of the oil shale will not kill the combustion front andsome initial space is provided. Naturally, chamber 10 must be ofsuflicient size so as to avoid shutting off or killing the combustionfront.

Once the shale has been ignited the process becomes self sustained askerogen is removed from the shale and the spent-shale or ash becomespermeable to the combustion supporting fluids.

In the embodiment of the invention shown in the FIGS. 1 and 2, thecentral borehole 3 is used as the recovery borehole and thedirectionally-drilled, converging boreholes 6 and 7, are used to supplycombustion-supporting fluids for the combustion front established at thetop of chamber 10. Also, if desirable, or necessary because of the lowquality of the shale, the combustion-supporting fluids may also containcombustible materials, such as hydrocarbon gases, in order to ensurethat combustion continues to occur in chamber 10 as it grows larger.However, in most cases, the oil shale itself will provide suificientfuel for the combustion front and it will be unnecessary for theaddition of combustible-supporting materials with thecombustion-supporting fluids injected into the chamber via the severalboreholes 6 and 7, respectively. It may be also advantageous to injectwater with the combustion supporting fluids. Water will be converted tosteam, resulting in a more uniform heat distribution across thecombustion front and lower the reaction temperature thus increasingyield. It also will recover heat from the burned out region, fillingthis area with water and in this manner preventing accummulation ofhydrocarbon products below the front, thus improving the effectivenessof the process.

Referring now to FIG. 2, wherein the process of this invention has beenemployed for a period of time, the changes occurring in the reservoircan be seen. Quite generally, the combustion front 11 originallyestablished at the top of chamber 10 has expanded horizontally toencompass a much larger portion of the reservoir, burning furtherradially outwardly from borehole 3 along the curving,upwardly-diverging, lower portions 8 and 9 of boreholes 6 and 7.Further, this burning of the oil shale has greatly enlarged chamber 10so that it encompasses a much larger portion of the reservoir as can beseen by reference to the dotted lines in FIG. 2 which represents theoriginal conditions in the reservoir prior to the expansion of acombustion front 11. The growth of the combustion front adds greatly tothe economy of this process since fewer wells are required to treat thesame volume of the reservoir 2. As shown in FIG. 2, chamber 10 hasbecome greatly enlarged and is filled with spent shale ash 12 left fromthe vertical, upward-burning of the combustion front. This provides anatural gravity separation of the spent shale from the unburned shale inthe vicinity of the combustion front which facilitates the verticalmovement of the front. However, since the chamber 10 provided for theinitial expansion of the shale caused by the heating, the enlargedchamber 10 in FIG. 2 does not require additional space to avoid highlycompressive loads across the combustion front 11. Further, the spentshale ash 12 from which the kerogen has been removed is gas permeablealong the bedding planes and through cracks in a direction perpendicularto the bedding and the combustion-supporting fluids can flow from thelower portions 8 and 9 of boreholes 6 and 7 respectively, through theash 12 towards and along the combustion front 11.

In addition, another substantial advantage is gained by this processwhich is the ability of the combustion-supporting fluids to travelgenerally along the laminar bedding planes of the unspent shaleformation to combustion front 11. This is important since as the shaleis heated, it tends to spall oif from the ceiling of chamber 10 as alaminant which allows the combustion-supporting fluids to be present onboth sides of the laminar bedding planes of the shale as it is beingconsumed by the combustion front 11 which materially facilitatescombustion.

As combustion front 11 moves vertically upwardly through the shalereservoir 2, it converts a substantial quantity of the structurelessorganic material or kerogen to vaporous hydrocarbon products which, inthe embodiment of the invention shown in the drawings, travel verticallyup through borehole 3 with the other products of the combustion from thecombustion front 11. These products can subsequently be recovered bysuitable condensing and separating equipment.

It should also be appreciated that a plurality of boreholes can be usedin carrying out this process and actually it is probably desirable tohave at least four directionally-drilled, converging boreholes in thetarget area which is penetrated by a central borehole for the recoveryof the gaseous eflluents from the combustion. In some cases, 6, 8 ormore converging boreholes Will be used depending on conditions.

In comparing FIG. 1 with FIG. 2, it can be seen that one of the novelfeatures of this invention is that the combustion front extends at alltimes from injection well(s) to production well(s) and continues toincrease in size and moves up through the shale formation therebyexposing more and more of the oil shale reservoir to the combustionfront which, of course, increases total recovery and the volume treated.The actual growth of the combustion front as it proceeds vertically upthrough the shale reservoir 2 is limited by the degree of divergenceobtainable between the upper portion of boreholes 6 and 7 and this inturn, is limited somewhat by the thickness of the shale formation.Generally, the process works exceptionally well in oil shale reservoirshaving thicknesses from 300 to 600 feet or greater.

The minimum thickness of the shale required to carry out the processwill be predominantly controlled by eco- 7 nomic reasons which depend onthe kerogen content of the shale and the depth at which these beds areencountered.

Exploitation of thinner shale reservoirs, e.g., less than 100 feetthick, may impose some limitations on the use of this process due to thelimited degree of curvature obtainable by directional drilling. This isnot thought to be a serious limitation, however, and possibly could beovercome by less conventional drilling techniques such as right angledrilling.

' It has also been proposed to drill a cluster of overlapping patternswhich are simultaneously ignited. Each pattern would include at least aneffiuent production borehole and a closely spaced combustiorvsupportingfluid injection borehole. The combustion area of each pattern wouldemanate upward from the terminate ends of the said boreholes in themanner of an inverted cone. Although each pattern would singularlyexploit only one third of the proximate layer of oil-shale, a muchgreater yield may be realized by causing the combustion fronts ofseveral patterns to grow together at some vertical distance above theterminal depth. This distance will depend on the amount of overlap ofthe individual patterns.

EXAMPLE As an illustrative example of the process of the presentinvention, several laboratory experiments have been carried out asdescribed below:

A 44-inch long, 4-inch diameter massive oil-shale core, which has beendrilled perpendicular to the bedding planes, is cemented inside a thinwalled 4-foot long, 4 /2- inch diameter steel tube which in turn ismounted inside a high pressure vessel. Two parallel /2-inch diameterholes, with 2% -inch distance between centers, are drilled lengthwisethrough the core serving as injection and production wells. At one endof the core, two Az-inch stainless-steel tubes are cementedapproximately 2 inches inside the /2-inch holes to provide connectionsfor the injection and production lines. This side of the tube isequivalent to the surface under actual field conditions. At the otherend, the two /2 -inch holes are in communication through a one-inch longcylindrical space filled with crushed oil shale. The crushed material iskept in place by a three-inch long oil-shale core plug. A 1%-inchdiameter hole is drilled through the center of this plug in which astainless-steel heater-well is inserted extending through the crushedmaterial approximately one-inch into the oil-shale core. This side ofthe tube is equivalent under actual field conditions to the bottom ofthe bore holes which are in communication through a common chamber orcavern as indicated in FIG. 1. For accurate control during theexperiments, nine thermocouples are mounted along the axis of the coreat equal distances of five inches.

After packing off both ends, an electric heater is inserted into theheater well and the pressure in the pressure vessel is increased toapproximately 200 p.s.i. A combustion supporting gas, in this case airor a mixture of 50 percent oxygen and 50 percent nitrogen is circulatedthrough the core by injecting into the /2-inch injection well atpressures ranging from 125 p.s.i. to 175 p.s.i. Initially the gas passesthrough the crushed oil-shale and is produced back through the /2 -inchproduction well. Prior to ignition, the combustion supporting gas isinjected and produced at a low rate equivalent to approximately 0.2cubic feet per minute. After ignition, the flow rate is increased andmay vary from 0.4 to 0.6 cubic feet per minute. Ignition occurs when thetemperature reaches 1000 to 1200 F. and a combustion front starts tomove through the core at a rate of about 3 inches per hour,countercurrent to the flow of the combustion supporting gas in theinjection well. The temperature of the front is approximately 1200 F.and sometimes increases to 1600 F. which can be reduced by the injectionof water. Usually an experiment is terminated 10 to 12 hours afterignition.

Production of exhaust gas, oil and water is carefully measured. Oilproduction depends greatly on the kerogen content of the oil shale whichhas varied from 12 to 40 gallons per ton Fisher assay. The gravity ofthe produced oil ranges from 18.0 API to 32 API, the lower gravity oilbeing produced during the initial stages of the experiment. Respectiveviscosities range from 40 to 7 centipoise. After terminating anexperiment, the core is lifted from the pressure vessel, the steel tubeis cut and together with the cement carefully removed. Inspection of thecore shows that the spent shale behind the front become very friable andflaky, displaying a high degree of permeability along the beddingplanes.

It should also be observed that the one-inch space filled with crushedoil-shale is not intrinsic to the experiment. In one particularexperiment, a space of only inch filled with sand grains was used. Theexperiment proceeded in the same manner as those described above, theonly difference being that ignition time was increased fromapproximately two hours to nine hours.

As can be appreciated from the above description, this novel approach tothe in situ combustion of oil shales solved many of the problemsexisting in the prior art processes and represents a practical solutionto in situ combustion of oil shales.

I claim as my invention:

1. A process of recovering hydrocarbon products from subterranean, oilshale formations which comprises the steps of:

(a) drilling at least two spaced boreholes into a subterranean, oilshale formation in a manner that their lower terminal ends approachconvergence substantially parallel to the bedding planes of theformation and adjacent to the bottom of said oil shale formation;

(b) removing sufiicient oil shale in the area of convergence to form ashale expansion communicating at all times with said lower terminal endsof said boreholes;

(c) establishing in situ combustion within said chamber;

(d) supplying a combustion-supporting fluid through at least one of saidboreholes to maintain said in situ combustion within said chamber and tocause a combustion front generally parallel to the bedding planes of theformation to move vertically and upwardly through said formationgenerally perpendicular to the bedding planes of said formation, thelateral dimensions of the combustion front being controlled by theconverging spaced boreholes and the combustion-supplying fluid beingsupplied generally along the laminar bedding planes of the unspent shaleformation and transverse to the movement of the combustion front; and

(e) recovering vaporous effluent from said formation produced by said insitu combustion through at least one of said boreholes.

2. A process according to claim 1 including the step of treating thevaporous effluents recovered from said formation to recover usefulhydrocarbons.

3. A process according to claim 1 including the step of casing theboreholes drilled into the oil shale formation only to a depth abovewhere they traverse said formation.

4. A process according to claim 1 including the step of drilling acentral borehole and at least four boreholes laterally spaced from saidcentral borehole so that their lower terminal ends approach convergencewith the lower terminal end of said central borehole.

5. A process according to claim 4 including the step of recovering thevaporous effluents from said formation produced by the in situcombustion through said central borehole and supplyingcombustion-supporting fluid to said in situ combustion through said fourboreholes laterally spaced from said central borehole.

'6. A process according to claim 1 including the step of combining acombustible material with said combustionsupporting fluid forfacilitating the in situ combustion.

7. A process according to claim 1 including the step of combining Waterwith said combustion-supporting fluid for further improvement of the insitu combustion process.

References Cited UNITED STATES PATENTS 2,788,956 4/1957 Pevere et a1.166-11 2,970,826 2/1961 Woodruff 166-11 X 3,001,775 9/1961 Allred 166-11OTHER REFERENCES Lombard: Recovering Oil From Shale With NuclearExplosives, Journal of Petroleum Technology, August 1965, vol. XV, No.8, pp. 877-882.

STEPHEN J. NOVOSAD, Primary Examiner US. Cl. X.R. 166-259, 261, 247

